Synthesis and crystal structure of RMgSi 2 compounds ( R=La, Ce, Pr, Nd), a particular example of linear intergrowth

Abstract

A new series of rare earth compounds with stoichiometry RMgSi 2 ( R=La, Ce, Pr, Nd) is reported. The single crystal X-ray diffraction showed that CeMgSi 2, which melts congruently at 1200 °C, crystallizes in a new tetragonal structure type ( I4 1/ amd, tI32, a=4.2652(4) Å, c=36.830(4) Å, Z=8; w R 2=0.042 (19 parameters, 393 F 0 2), R 1=0.018 (297 F 0>4 σF 0). The crystal structure of CeMgSi 2 can be formally built up by alternating along the z direction four CeMg 2Si 2-type CeMg 2Si 2 slabs with four AlB 2-type CeSi 2 slabs, one after the other. The structural model obtained from a CeMgSi 2 single crystal has been confirmed for the La, Pr and Nd homologous compounds by means of Rietveld refinement. The trend of the unit-cell parameters, plotted versus the R 3+ ionic radius, shows a linear behaviour, which strongly suggests a trivalent state for the Ce atoms. An analysis of the features of this new structure is reported, in comparison with the other known CeMg 2Si 2/AlB 2-type linear intergrowth compounds.